1. Field of the Invention
This invention pertains to intervertebral fusion. More particularly, this invention pertains to an implant to facilitate fusion between two vertebrae.
2. Description of the Prior Art
Back pain is extremely debilitating. Individuals suffering from severe back pain are frequently precluded from the full enjoyment of life including gainful employment and leisure activities. In addition to the substantial human costs associated with back pain, society bears a substantial financial cost. Lost employment time adversely impacts on productivity as does medical insurance costs.
Frequently, the cause of back pain is traceable to diseased disc material between opposing vertebrae. When the disc material is diseased, the opposing vertebrae are not adequately supported.
In order to address back pain resulting from disc disease, prior art surgical techniques have been developed to fuse the opposing vertebrae. Such techniques include removing the diseased disc and packing the disc space with bone. The intent of such a procedure is for the packed bone to grow together and fuse with the bone of the opposing vertebra. If successful, the two opposing vertebrae are now rigidly linked thereby avoiding intervertebral instability.
In fusing opposing vertebrae, the prior art has developed surgical techniques and apparatus to facilitate interbody fusion as well as to permit stabilization of the vertebra while the fusion process is occurring. To this end, surgical implants have been developed.
An example of a surgical implant for facilitating interbody fusion is shown in U.S. Pat. No. 5,015,247 to Michelson dated May 14, 1991. Michelson uses a circular cross-section cylindrical implant which is of uniform diameter throughout its length and which includes an external thread. The implant is hollow and has holes formed through the cylindrical wall of the implant. The implant is placed within a prepared site between the vertebrae. The prepared site is a bore formed through the disc material as well as partially formed through the end plates of the opposing vertebrae. The implant is threaded into the bore and packed with bone chips or the like.
Another example of an interbody fusion device is shown U.S. Pat. No. 4,834,757 to Brantigan dated May 30, 1989. The Brantigan device is a parallelepiped plug which is forced into a complementarily shaped cavity formed between opposing vertebrae.
Prior art interbody fusion devices are not trouble free. For example, prior art devices suffer from uncontrolled subsidence of the device into the vertebral body. By subsidence, it is meant that after the implant is placed between the opposing vertebra, the implant migrates into the vertebral body. Also,. in many prior art implants, direct bone apposition only occurs on two surfaces. In addition, unwanted invasion of disc or cartilage material into the implant may occur upon insertion. Such prior art devices typically have minimal surface area contact with the end plates of the vertebra. In addition to the above, such prior art devices have a geometry which prevents close placement when two implants are placed in a side-by-side relation within a common disc space. A prior art device to increase the density of implant placement is shown in U.S. Pat. No. 5,055,104 to Ray dated Oct. 8, 1991. In that patent, the device is a helical thread. Two such devices are placed side-by-side with the threads intermeshing.
It is an object of the present invention to provide an implant for use in interbody fusion. It is a further object of the present invention to provide such an implant which has reduced subsidence.